Solid State Replacement for the mid 1970's Williams Electromechanical Match Unit


The match unit on my 1976 Williams Aztec had a badly worn shaft/bushing and subsequently the wiper contacts didn't track the printed circuit board (PCB) properly.

I knew it wouldn't be easy finding a good replacement so I decided to design and build an electronic replacement.

What I came up with was a 16F628 PIC controlling 2 strings of 10 triacs which substitutes for the 2 pole,10 position function of the original electromechanical unit. I like to keep my pinballs as original as possible but in this case I'd rather have the correct match functionality rather than a faulty one. Also,others might be interested as well.

This project won't be for everyone because it uses quite a few components and is moderately complex but if you can source the components economically and have the required skills then it might be useful to you.

I also added an extra triac which serves the function of the additional bi-stable switch which has the "Change" function in the Aztec. This switch alternates from on to off and back again every time 10 points is scored on the Aztec (minimum score).

Here's a picture of the original match unit.

As can be seen from the following pictures , I've used 2 printed circuit boards to mount all the components. They mount on the original match unit frame and are attached using threaded spacers. Just strip out the mechanical bits and the coil and attach the assembled PCBs. No extra holes need to be drilled because I've used the same mounting holes as the original PCBs. This means that the whole assembly mounts in exactly the same location as the original. All the connecting wires along the top solder to terminals on the new PCBs in the same alignment as the originals.

Here's a PIC and lamp switching side view of the new match unit. Notice that there are 3 momentary push button switches (one is mostly obscured). One switch is reset for the PIC,one is to simulate input pulses and the other one is for "lamp test". When the "lamp test" switch is pressed,all the match lamps are turned on so that they can be checked as working OK.

The coil wires and the "Change" switch wires also reach to their corresponding terminals on the PCBs. Two additional wires are needed to connect to 6.3VAC on a nearby GI lamp socket via a 250mA fuse. This supplies power to a full wave voltage doubler and regulator which supplies +5V to the electronics.

Here's a partial coil switching side view of the new match unit.

The pictures of the PCBs show the first prototype. I've modified the layouts slightly and will make the second and hopefully final prototype soon. If there's enough interest I'll get the PCBs made by a PCB manufacturer and sell programmed 16F628 PIC chips to anyone that wants them. As well as making the relevant documents and software listing available on line. My intention is for the project to be public domain so that anyone that wants to make one can.

I work on this when I get the chance so I don't expect to have the second prototype finished until the new year.

Let me know at jennyah2@bigpond.com if you're interested.

Here's another view inside the Aztec backbox.

rgp Gruntcake 3rd. November 2008

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